Chlorine-Initiated Oxidation of α-Pinene: Formation of Secondary Organic Aerosol and Highly Oxygenated Organic Molecules

The oxidation of alpha-pinene, the most abundantly emitted monoterpene, is an important source of secondary organic aerosol. Previous work has focused on alpha-pinene oxidation by hydroxyl (OH) radicals or ozone (O-3), yet the effect of chlorine radicals (Cl) as oxidizing agents has not received much attention. Recent ambient measurements suggest that Cl could substantially affect SOA formation in continental as well as coastal regions. In this work, we conducted environmental chamber experiments on the Cl-initiated photo-oxidation of alpha-pinene leading to the formation of oxidized products including highly oxygenated organic molecules (HOM) and SOA. We use a Filter Inlet for Gases and Aerosols mounted onto a High-Resolution Time-of-Flight Chemical Ionization Mass Spectrometer (FIGAERO-CIMS) to monitor gas- and particle-phase products and to investigate their gas-particle partitioning. We identify over 600 species including 197 chlorinated and nonchlorinated HOM, which are up to six times more abundant in the particle phase than in the gas phase. We also find that SOA yields are enhanced under low NOx conditions. Overall, our work suggests that chlorine-initiated oxidation of alpha-pinene can substantially impact tropospheric SOA formation given the fast chlorine chemistry and the formation of low volatility HOM which contribute to high SOA yields.